1. Technical Field
This invention relates to the art of making iron-based cutting tools and, particularly, to a coating mechanism that promotes long tool life at high machining speeds.
2. Description of the Prior Art
Various coatings have been employed to increase the tool life of cutting tools made of tool steels. These coatings have principally consisted of carbides and nitrides applied with a variety of techniques. However, such coatings do not exhibit the unusual wear characteristic of titanium diboride. Titanium diboride has an extremely high vickers micro-hardness value, typically about 3600, which is not only considerably higher than other borides but also substantially higher than other carbides or nitrides. Titanium diboride is also particularly noted for its high density, e.g., 88% of theoretical density, a low resistivity of 30 micro-ohms centimeters, a high strength of about 40,000 psi, and a coefficient of thermal expansion which is about 8.1.times.10.sup.-6 at the temperature range of 20.degree.-800.degree. C.
One of the most troubling problems associated with deployment of titanium diboride on tool steels is that of adhering it thereto. It is known that titanium diboride can be diffused into steel, but this is disadvantageous because it would not obtain the best metal boride for wear resistance. In U.S. Pat. No. 3,029,162, a disclosure was made as to how to first coat a layer of titanium onto a steel substrate and thereafter diffuse the boride into the titanium to obtain a coating of titanium diboride. Unfortunately, this technique requires multiple layers and is not easily controllable to obtain a desired thickness of titanium diboride and, most importantly, results in a sublayer of pure titanium which is not only expensive but disadvantageous. In another related patent, namely, U.S. Pat. No. 4,411,960, a slurry of molten titanium metal was coated onto a steel substrate and thereafter quenched in a boride salt bath to promote diffusion of the boron into the titanium. For the reasons recited above, this multiple step diffusion process is not desirable.
An attempt was made to secure titanium diboride or similar type compounds to a steel substrate by the use of soldering (see U.S. Pat. No. 3,160,480). A soldering substance, particularly disclosed as being Ni--Fe--Co alloy, was used which, when heated to the temperature range of 1100.degree. C., operated and functioned much as a soldering material to attach the titanium diboride thereto. Unfortunately, this is undesirable because in the use of steel substrates, heating to soldering temperatures may destroy the heat treatment characteristics of steel previously promoted in the material and would require additional reheating to reestablish such characteristics.
Attempts have been made to deposit titanium diboride onto a steel substrate by chemical vapor deposition (see U.S. Pat. No. 4,237,184). The deposition is generally effected at a temperature between about 850.degree.-1200.degree. C. The starting gaseous product is typically two halogenides and hydrogen. Unfortunately, the requirement for such high temperature heating destroys any previous heat treatment that has been promoted in the steel substrate and would require reheating again.
Sputtering is another deposition technique which has been mentioned in connection with a variety of other materials other than titanium diboride. Applicant is unaware of any attempt to apply titanium diboride to steel substrates by sputtering; this may in part be explained by the difficulty in obtaining an ultra-clean surface on steel substrates to receive the condensate of the sputtered material.
Accordingly, a primary object of this invention is to provide a method by which titanium diboride may be deposited in a very adherent manner in a relatively thin coating to achieve a highly wear resistant coating on steel substrates which has its own modulus or rigidity and does not require special preparation or treatment of the substrate after such deposition.